Ecology - Habitats

Microhabitats

People often think of habitats on a relatively large scale - deserts, coniferous forests, sclerophyll forests, grasslands, bogs, alpine peaks and so on. Such broad habitats are important when talking about lichens but it is equally important to look at a smaller scale at the same time. To a lichen a shift of as little as a centimetre may mean a shift to dramatically different conditions where, perhaps, survival is not possible. If a shift of a few centimetres means a shift to a different set of conditions, that also means a shift to a different micro-habitat. When looking for lichens in the field it is important to be aware of micro-habitats and realize that sometimes, while taking just a few paces, you may have crossed a dozen different micro-habitats. The aim of this page is to point out a variety of microhabitats but as you walk around in town or country you should be able to spot many more for yourself.

Consider the boulders shown in this photo . Such boulders are fairly common in many farm paddocks and natural grasslands and, almost invariably, you'll see a mix of lichens and bryophytes growing on them. The lichens growing on these boulders are different to the lichens growing on the surrounding soil. From a lichen perspective the boulder differs from the surrounding soil. It has a different texture and a different chemistry. Whereas the grasses (and occasional shrubs) of the surrounding area give some shade to the soil, the boulders have no shading grasses or shrubs. The soil soaks up water, the boulders don't. Each boulder is an example of a micro-habitat since it is quite different to the surrounding area. The boulders also heat up more quickly than the surrounding soil and hold that heat longer. The photo shows a scene in Namadgi National Park in the Australian Capital Territory. For at least a part of each winter the grassy area will be under snow and at many other times of the year there will be morning frost in this valley. The larger boulders, at least, will either be above the ground level cold air layer or they would warm up before the soil does. That means that any overnight frost on the boulders will be thawed into water that can be used by the boulder lichens while the soil lichens are still frozen and dormant. When it rains some of the rain falling on a boulder will be absorbed by the lichens or bryophytes on it but often there will also be some runoff. The soil around a boulder will be moister than that further away since the area near the boulder receives direct rainfall as well as runoff. During heavy rain the additional water from runoff will not make a great difference but during light rain (or fog) the runoff can make a large difference, even dominating the direct rainfall in the immediate vicinity of the boulder. That is especially so when boulders are large and very broad in relation to height. That shape provides a large 'catchment' area and the potential for substantial runoff. In arid, low-rainfall areas you will often see richer lichen growths in the microhabitats immediately around the bases of such boulders. Of course, around a boulder that's taller than broad the ground on different sides gets different levels of shading during sunny days, creating yet further soil microhabitats.

Lichen covered rock near Cooma, NSW
(click to enlarge)

This isolated rock (right) in a grassy area near Cooma in New South Wales is an example of a smaller microhabitat and supports species that you would not find on the surrounding soil. In this case the microhabitat is still easy to spot because the rock stands out, but what about this photo ? It shows a largely bare area of ground, mostly soil with a few pebbles. There are a few Xanthoparmelia thalli with the largest, in the lower left, being about two centimetres in diameter. All these thalli were growing on pebbles and in most cases it's easy to see this if you look closely, because the pebbles are generally only partially covered. The largest thallus has completely covered its pebble substrate so it would be understandable, at first glance, to think that thallus was growing on soil - as do the thalli of some Xanthoparmelia species. However, in this case the pebbles constitute a microhabitat distinct from the surrounding soil and one that is host to a rock-loving Xanthoparmelia species.

This photo shows a lichen-free zone beneath a commemorative plaque on a boulder otherwise well-covered by lichens (in particular a greenish Xanthoparmelia). The plaque is likely to be a copper alloy with copper leaching from the plaque during rain. Most lichens cannot tolerate copper and so there is a lichen 'desert' in the run-off zone downhill from the plaque. This photo shows a Xanthoparmelia species growing on the metal lid (about 15 centimetres square) over an inspection hole in a Canberra suburb. While there is extensive growth on the metal there is no Xanthoparmelia on the concrete around the lid. Xanthoparmelia propagules are as likely to have landed on the metal as on the concrete, but few species of Xanthoparmelia tolerate alkaline substrates, such as concrete. Thus, within the small area shown by this photo there are two markedly different microhabitats - one of which allows this Xanthoparmelia to thrive.

Rough areas, such as rough bark or cracks in rocks, are more likely to trap propagules and water than are smooth surfaces - all else being equal. Thus it is not surprising to find lichens well-established in rough areas but absent (or nearly so) from nearby smooth areas that have the same aspect, chemistry and so on. This photo shows Acarospora citrina (yellow) and a Caloplaca (orange-brown) growing along cracks or surface irregularities. Here is part of a tiled area around an outdoor memorial statue in Canberra. The lens cap has a diameter of five centimetres and you can see a number of brown lichen thalli within the depressed areas of a paving tile. It's certainly true that in this area the depressions within the tiles and the gaps between tiles would hold propagules and water better than the smooth, raised tile areas but there is also another factor to consider. The raised areas would experience more scuffing and abrasion as people walk around this memorial and, while growth is possible on the higher areas, it is more likely to be damaged by foot traffic. Here is a closer view, showing Ramboldia petraeoides (the extensive brown area, with the yellowish lichen being a species of Caloplaca) within one of the tile depressions. Here is a close look at a bitumen path in suburban Canberra. You can see a good growth of an orange-brown Caloplaca in the lower areas. A little below and to the right of the photo's centre you can see a large group of darker apothecia. Each apothecium is no more than about a millimetre in diameter. You'll find a good example of the influence of foot traffic in the CLADONIA FURCATA CASE STUDY. Normally Cladonia furcata thalli would develop upright podetia fairly early but in this case constant foot traffic keep the thalli squamulose.

Many lichens grow on trees (and other plants). A particular tree may have numerous microhabitats - north side, south side, rough bark, smooth bark, trunk, branches, leaves. Many lichen species are found growing on leaves but not on other parts of trees or shrubs and these are the FOLIICOLOUS LICHENS. Hence, as far as some lichens are concerned, leaves are microhabitats that they can inhabit but the rest of the tree is uninhabitable. There are microhabitats even on leaves, for example upper and lower surfaces. Some foliicolous species will be found growing only on one side, never on the other. Even on one surface there can be different microhabitats, for example damaged and undamaged areas. Here is a well-developed bright green thallus of Strigula smaragdula growing on a dark green background - the surface of a live leaf on a tropical tree. This lichen establishes itself at damaged areas, such as the hole through the leaf shown in that photo. Once established the thallus expands into undamaged areas but a wound of some sort is necessary for establishment of the lichen.

Paraporpidia leptocarpa, on a paved area alongside the Cryptogam Herbarium, Canberra
(click to enlarge)

This photo shows part of the cryptogam herbarium building within the grounds of the Australian National Botanic Gardens in Canberra. The upper level extends a little over a metre beyond the wall of the lower level and along the whole length of that wall. Next to the wall is a broad, paved area with shrubs and trees further to the right. The building and the nearby vegetation tend to keep windblown rain (regardless of direction) from reaching the area near the wall and of course the overhang blocks vertical raindrops. Hence the metre-wide band of pavement closest to the wall gets very little rain during the year, unlike the exposed part to the right. The latter area supports a small variety of crustose species, whereas lichens are virtually absent nearer to the wall. You can see a closer view of part of the paved area in this photo , with a little of the building's wall in the photo's upper left. A number of crustose thalli are easily visible in the right half of the photo, but none on the left, and there are many more thalli to the right but most are rusty brown and so don't show very well against the pavement colour. If you imagine a straight line drawn between the two small red dots (one near the centre of the photo's upper edge and the other a little left of centre on the lower edge) you'd be marking the extent of the overhang. There is a small amount of pedestrian movement through this area each day and botanic gardens' vehicles drive through it from time to time. The photo to the right shows a closer view of one of the lichen thalli that appeared in the previous photo.

Microhabitats in hot, arid areas

LICHENS IN ARID AREAS face quite stressful conditions and some will survive (or thrive) only in particular microhabitats rather than at random locations. When vascular vegetation is present it can create shade, provide protection from drying winds and abrasive wind-blown sand grains. Even a grass tussock can provide several microhabitats, as shown in the SPANISH GRASS CASE STUDY. Near a tussock base features such as light levels and soil chemistry can be distinctly different from what is found beneath the tussock's outer extremity.

Cacti can be found in some fog deserts, that is, coastal deserts which gain much of their water from fog. Cactus needles provide excellent condensation points for water vapour and so play a significant role in trapping fog moisture. Where the spines point downward water would be gathered and concentrated on the spines into larger drops which would then drop to the ground, there to be absorbed by the roots. One researcher observed such cacti in southern Chile and noted that epiphytic lichens grew on the cactus spines, but nearer the spine tips than their bases. In that position a lichen would maximize its uptake of water whereas a spine base would constitute a drier microhabitat. Small rocks and large boulders also provide shade and protection. As noted early on this page rocks may also concentrate water around their bases.

In the absence of sheltering plants or rocks, orientation can provide some protection in exposed areas. In the southern hemisphere an exposed, north-facing slope would be sunlit for much of the day and therefore would tend to experience higher surface temperatures than an exposed south-facing slope - with the reverse in the northern hemisphere. Thus, if you walk up a north-facing slope, over the top and then down the south-facing slope you would be passing from one microhabitat to another in the space of a few metres. It should also be clear that the steeper the slopes the more pronounced would be the differences between north and south faces. All else being equal, the more protected slope should be able to support a greater variety of species and also a greater amount of lichen growth. The results from a study of the lichens at three desert sites in south-west North America give some examples of the differences that can be found. There were 17, 21 and 47 species on the three north-facing slopes and the corresponding species numbers on south-facing slopes were 3, 6 and 17. The estimated lichen coverages (in square metres per hectare and rounded to whole numbers) were 13/3, 260/3 and 813/71 (the numbers before the slanting line being for the north-facing slopes, the numbers after being for the south-facing slopes - and the three pairs are given in the same order as the species counts). Finally, estimated biomasses were 1.2/0.4, 14.3/0.2 and 38.3/4.9 (in grams per square meter, rounded to one decimal place and with presentation order as before).

Water-holding depressions were mentioned early on this page - but not all depressions have equal effects. On boulders, such as those shown earlier with Acarospora and Caloplaca thalli, depressions may trap water after rainfall, especially useful when rainfall has been very light. In areas with little rain but much fog boulders that project above the soil surface may also intercept moving fog, induce condensation of water and then trap it in crevices. The authors of the paper listed in the next reference button looked at vehicle tracks in an area of the Namib Desert where rainfall is rare but fog is common. The vehicle tracks were 10 to 15 centimetres deep and oriented north-south. Conditions in the tracks were compared with those on the undamaged surfaces a metre away. It was found that, when compared to undamaged surfaces, fog deposition was markedly lower in such tracks but temperatures higher from sunrise to about mid-afternoon. On average, the temperature difference was 2 degrees Celsius by 1pm. Rates of post-sunrise evaporative water loss were initially similar in tracks and undamaged areas. Two hours after sunrise the loss rate was higher in the vehicle tracks, though the rate slowed after that and after another two hours was similar to that of undamaged areas. These (and some other) differences are enough to preclude some soil lichen species re-colonizing the tracks so that for such species vehicle tracks are inhospitable microhabitats.

An illustration of microhabitats defined by wind and fog is shown by the diagram on the right, which shows a hillock in the central Namib Desert. The hillock is orientated south-west to north-east, with the south-west slope to the left, and was 2.3 kilometres inland. Within this particular area there were many hillocks, with mean heights between 5 and 8 metres. The base length of this particular hillock is about 300 metres and the vertical scale is highly exaggerated in this sketch since the hillock rises to a maximum height of five metres. Grey indicates the area dominated by Teloschistes capensis cushions (with less dense coverage in the dotted area), blue the area occupied mainly by foliose lichens and black the area occupied mainly by crustose lichens. The left (ie, south-west) slope faces the sea, hence also the fogs that move inland and so gathers much of the moisture - ideal conditions for the fruticose Teloschistes. The inland (right-hand) slope receives the least water and crustose species are best-suited for such areas. On this hillock the biomass on the south-west slope was close to 350 grams (dry weight) per square metre, on the top of the hillock the amount was about 60 grams and on the north-east slope about 30 grams. A number of study sites in the central Namib showed this pattern of fruticose or foliose dominating on the coastal aspect and crustose on the inland aspect. In the central Namib there are two main wind directions - south-west winds from the sea moisten the area throughout summer and periodic hot, dry north-east winds are a characteristic of winter.

The sides or undersides of translucent rocks or pebbles lying on the ground provide sheltered habitats for various lichens. Light can get through such stones but some of the heat and intensity are dissipated as the light rays pass through. Intense light can be detrimental to some lichens. Water condenses on the stony surfaces, for example as dew or from fogs, and the droplets will run down and around to the underside. A striking example of the use of such a microhabitat is given by Peltula inversa, known from Namibia. Thalli are found around quartz pebbles, near the pebble-soil boundary, with very little of the thallus above the soil surface. On the left of the following diagram is part of a quartz pebble (in gray), the soil (orange-brown) and a Peltula inversa thallus (in black). The photobiont layer (shown in green) is composed of cyanobacteria and is positioned near the pebble surface. A light ray (shown in dark blue) has entered the pebble, been refracted or reflected a few times at internal inhomogeneities and finally reached the thallus and the photobiont cells. The light intensity is still enough to allow effective photosynthesis. Also shown is an apothecium (lighter blue) on the soil surface.

The undersides of rocks or pebbles are also protected microhabitats and to the right of the same diagram is another translucent pebble with a couple of lichen thalli (in black) hugging the pebble's underside. Light rays entering the pebble will travel through along various pathways. Amphoridium caesiopsilum, Lecanora terrestris, Sphaerothallia desertorum and Sphaerothallia lacunosa are some species found growing on the undersides of translucent rocks in Central Asia. Apart from acting as water catchments and attenuators of light or heat rocks and pebbles also give some protection against the erosive effects of wind-blown sand grains.

The upper surfaces of any pebble, translucent or opaque, can collect water which then runs down to the underside, creating a moist niche below. The Mt. Isa region of western Queensland has a hot, semi-arid climate. In at least parts of the area few lichens are visible as you walk across the pebble and rock-strewn ground. But, look under the numerous opaque pebbles or rocks and you can finds many lichens, some growing on the pebble or rock surfaces, others on the soil.